1. Field of the Invention
The present invention relates to the control of a motor, especially relates to the control of a stepping motor used in an image scanner of global range of scanning speed.
2. Description of the Prior Art
In the application of an image scanner, the resolution is preferable to be related to the velocity of the movement of the scanning head. Because when the fixed scanning speed is utilized, the higher the resolution is, the more time the scanning head takes to transmit the acquired data. If the spec of the image scanner is 300.times.600 dpi (dot per inch), and the user can choose the resulted image data from 75 dpi to 600 dpi, the speed of the scanning head of the former is preferred to be 8 times that of the later. The relation between torque and speed of the stepping motor is approached in FIG. 1A. It is obvious that when the speed of the scanning head increases, the torque of the stepping motor decreases, so the speed of the scanning motor is available only in a limited range.
When the speed of the stepping motor is beyond the scope of the available range, the problems of the stepping motor such as insufficient torque, resonance and acoustic noise are produced. The insufficient torque and resonance of the stepping motor cause the smearing interruption, jitter and vibration of the output image of the image scanner. The acoustic noise makes the user uncomfortable. For the phenomenon mentioned above, some application speed of the stepping motor is unavailable, and the designer of the image scanner can not make use of the stepping motor operating in every range of the velocity of the scanning head.
Because the stepping motor controls the movement of the scanning head of the image scanner, the characteristic of the stepping motor is essential for the performance of the image scanner. Take a four-phase stepping motor for example. It is obvious that, in FIG. 1B, the change of the currents in four stator windings N.sub.1, N.sub.2, N.sub.3 and N.sub.4 controls the rotation of the rotor 10. The phase difference of the currents in the first and second stator windings N.sub.1 and N.sub.2 is 180 degree in radiance, and the phase difference of the currents in the third and fourth stator windings N3 and N4 is 180 degree in radiance too. The phase difference of the currents in the first and third stator windings N.sub.1 and N.sub.3 is 90 degree in radiance. The usual driving modes are unipolar, bipolar, 1 phase, 2 phase and 1-2 phase.
The configuration of the stepping motor used in the image scanner and the driver of the stepping motor are shown as motor 15 and driver 19 in FIG. 1C. The driver 19 is used to provide the current that drives motor 15. The driver 19 is usually a set of bipolar transistor or a set of MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Make use of the 1-phase-winding-model of a stepping motor, the circuit diagram of driver 19 and motor 15 can be expressed as that in FIG. 1D. The 1-phase-winding-model of motor 15 is shown as motor-model 100 in FIG. 1D and the driver 19 (unipolar driver) in FIG. 1C is shown as bipolar transistor 105 in FIG. 1D.
As shown in FIG. 1D, the motor-model 100 can be treated as an inductance L in series with a resistor R.sub.M The resistor R' is connected in series with the motor-model 100 to provide additional resistance for the control of time constant of stepping motor. Because the term R' in the time constant (L)/(R.sub.M +R')! can be substituted by the designer of the stepping motor, the control of the time constant is thus carried out. Referring to FIG. 1D, when the voltage on terminal V.sub.B is the voltage waveform 110 (FIG. 1E), the exiting current on the stator winding is of the waveform 115 (FIG. 1F), which is without consideration of back EMF (Electro-Mechanical Force). Because the exiting current on the stator winding determines the position of the rotor 10, and the duration from point a to b and point c to d are both time constant of the stepping motor, assume the time constant to be .tau., .tau. is essential to the stepping motor. When the back EMF is taken into consideration, the waveform of the exciting current in the stator winding is shown in FIG. 1G. It is clear that even when the driver is turned off, the current on the stator winding has not yet return to its minimum value. Thus the zero current recovery-time, i.e., the duration between point d and point c, is lengthened.
Though the resistor is connected to the stepping motor to avoid the shiver and acoustic noise, yet the current on the stator winding is reduced and the voltage-drop of the stepping motor is also reduced. For the reason mentioned above, the generated torque of the stepping motor become smaller due to the decrease of winding current, and the jitter and smearing interruption of the resulted image is produced. So the designer of the image scanner must make a trade off decision between time constant and torque of the stepping motor. Unless the stepping motor of excellent speed-torque characteristic is utilized, the global speed range of the scanning speed is impossible for the normal stepping motor under one fixed parameter.
Referring to FIG. 1 H, to decrease the back EMF, a Zener diode Z.sub.D is connected in series with the diode D. Thus the influence of back EMF is eliminated because the Zener diode Z.sub.D can provide shorter turn-off decay time. For all the aforementioned principles, the available speed range is confined to a scope because of the fixed parameter (resistor R.sub.M and Zener diode Z.sub.D). If the designer of the image scanner wants to utilize the stepping motor that can operate in a wide range of rotating speed under one fixed parameter, i.e., the stepping motor of excellent dynamic characteristic, the cost will be very high, and the manufacturer rarely makes the aforementioned stepping motor.